Non-linear time-of-flight mass spectrometer

ABSTRACT

A time-of-flight mass spectrometer has a first electrode, a second electrode spaced apart from the first electrode, a third electrode arranged between the first and second electrodes. The third electrode reserves a space for ions to travel between the first and second electrodes. The time-of-flight mass spectrometer further includes a sample probe disposed proximate the first electrode and adapted to hold a sample, and a detector disposed proximate the second electrode. The first electrode is adapted to be connected to a voltage source to cause a difference in voltage between the first and second electrodes to provide an electric field therebetween that changes non-linearly along an ion path between the sample probe and the detector for accelerating ions to be detected.

PRIOR PROVISIONAL APPLICATION INFORMATION

This Application is based on Provisional Application No. 60/384,343filed May 30, 2002, the entire contents of which is hereby incorporatedby reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The present invention was conceived during the course of work supportedby grant No. GM64402 from the National Institutes of Health and DARPAgrants NBCH1020007 and DABT63-99-1-0006.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a mass spectrometer in general and inparticular to a mass spectrometer that employs one or more spatiallynon-linear fields to accelerate ions from an ion source to a detector.

2. Description of Related Art

Mass spectrometers are instruments that are used to determine thechemical composition of substances and the structures of molecules. Ingeneral they consist of an ion source where neutral molecules areionized, a mass analyzer where ions are separated according to theirmass/charge ratio, and a detector. Mass analyzers come in a variety oftypes, including magnetic field (B) instruments, combined electric andmagnetic field or double-focusing instruments (EB or BE), quadrupoleelectric field (Q) instruments, and time-of-flight (TOF) instruments. Inaddition, two or more analyzers may be combined in a single instrumentto produce tandem (MS/MS) mass spectrometers. These include tripleanalyzers (EBE), four sector mass spectrometers (EBEB or BEEB), triplequadrupoles (QqQ) and hybrids (such as the EBqQ).

In tandem mass spectrometers, the first mass analyzer is generally usedto select a precursor ion from among the ions normally observed in amass spectrum. Fragmentation is then induced in a region located betweenthe mass analyzers, and the second mass analyzer is used to provide amass spectrum of the product ions. Tandem mass spectrometers may beutilized for ion structure studies by establishing the relationshipbetween a series of molecular and fragment precursor ions and theirproducts.

Alternatively, they are now commonly used to determine the structures ofbiological molecules in complex mixtures that are not completelyfractionated by chromatographic methods. These may include mixtures of(for example) peptides, glycopeptides or glycolipids. In the case ofpeptides, fragmentation produces information on the amino acid sequence.

One type of mass spectrometer is time-of-flight (TOF) massspectrometers. The simplest version of a time-of-flight massspectrometer, illustrated in FIG. 1A (Cotter, Robert J., Time-of-FlightMass Spectrometry: Instrumentation and Applications in BiologicalResearch, American Chemical Society, Washington, D.C., 1997), the entirecontents of which is hereby incorporated by reference, consists of ashort source region 10, a longer field-free drift region 12 and adetector 14. Ions are formed and accelerated to their final kineticenergies in the short source region 10 by an electric field defined byvoltages on a backing plate 16 and drawout grid 18. The longerfield-free drift region 12 is bounded by drawout grid 18 and an exitgrid 20.

In the most common configuration, the drawout grid 18 and exit grid 20(and therefore the entire drift length) are at ground potential, thevoltage on the backing plate 16 is V, and the ions are accelerated inthe source region to an energy: mv²/2=z eV, where m is the mass of theion, v is its velocity, z the number of charges, and e is the charge onan electron. The ions then pass through the drift region 12 and their(approximate) flight time(s) is given by the formula:t=[(m/z)/2 eV]^(1/2) D   (I).which shows a square root dependence upon mass. Typically, the length sof source region 10 is of the order of 0.5 cm, while drift lengths (D)ranges from 15 cm to 8 meters. Accelerating voltages (V) can range froma few hundred volts to 30 kV, and flight time are of the order of 5 to100 microseconds. Generally, the accelerating voltage is selected to berelatively high in order to minimize the effects on mass resolutionarising from initial kinetic energies and to enable the detection oflarge ions. For example, the accelerating voltage of 20 KV (asillustrated, for example, in FIG. 1) has been found to be sufficient fordetection of masses in excess of 300 kDaltons.

A profile of the acceleration potential in the source region 10 (shownin FIG. 1A) is shown in FIG. 1B. The potential in this embodimentdecreases linearly from a maximum value at the backing plate 16 (shownin FIG. 1A) to zero at the drawout grid 18 (Shown in FIG. 1A).

In recent years, the development of an ionization technique for massspectrometers known as matrix-assisted laser desorption ionization(MALDI) has generated considerable interest in the use of time-of-flightmass spectrometers and in improvement of their performance. MALDI isparticularly effective in ionizing large molecules (e.g. peptides andproteins, carbohydrates, glycolipids, glycoproteins, andoligonucleotides (DNA)) as well as other polymers. The TOF massspectrometer provides an advantage for MALDI analysis by simultaneouslyrecording ions over a broad mass range, which is the so calledmultichannel advantage. In the MALDI method of ionization, biomoleculesto be analyzed are recrystallized in a solid matrix (e.g., sinnipinicacid, 3-hydroxy picolinic acid, etc.) of a low mass chromophore that isstrongly absorbing in the wavelength region of the pulsed laser used toinitiate ionization. Following absorption of the laser radiation by thematrix, ionization of the analyte molecules occurs as a result ofdesorption and subsequent charge exchange processes. In TOF instruments,all ion optical elements and the detector are enclosed within a vacuumchamber to ensure that ions, once formed, reach the detector withoutcollisions with the background gas.

One of the performance criteria for a MALDI-TOF mass spectrometer is theresolving power. The resolving power represents the extent to which ionsof different m/z ratios can be distinguished from each other. Ideally,nearly infinite resolving power could be attained if all ions having thesame m/z ratio would arrive at the detector simultaneously. However,because MALDI generated ions are formed with a range of initial energiesand are extracted from the ion source over a range of startingpositions, the ions acquire a range of kinetic energies over a range oftimes and the resolving power is consequently diminished. Therefore, inorder to compensate for these variations in ion starting conditions andin order to attain sufficient resolving power, design features areincorporated in the Time-of-Flight spectrometer.

A number of techniques have been developed to improve the massresolution of time-of-flight mass spectrometers. The first majorimprovement to resolving power incorporated two design features thatimproved both mass resolving power and overall mass range. One of thedesign features was the development of a two-field ion source (Wiley, W.C., McLaren, I. H., Rev. Sci. Instrumen. 1955, 26, 1150-1157; Wiley, W.C., Science, 1956, 124, 817-820; Wiley, W. C. U.S. Pat. No. 2,685,035).Earlier ion sources used a single electric field for ion extraction thatimposed a tradeoff between energy and space focusing. FIG. 2A shows agraph of the voltage potential versus the length S₀ between the ionsource (backing plate) and the drawout grid or exit grid. The voltagepotential decreases linearly to reach zero volt at the exit grid,illustrated in FIG. 2A by a dotted vertical line. The focus positionlies at a distance of 2S₀ from the exit grid. The focus position isindicated on FIG. 2A by a vertical line.

In order to maximize energy resolution, high electric field strength wasused to accelerate the ions to their final velocity quickly. However,this required an axially short ion source geometry. In order to achievea space focus condition the detector is placed also at a short distance(2S₀) from the ion source. Hence, the time of flight was not long enoughto achieve mass separation. The total time-of-flight could only beincreased by either lowering the electric field strength, consequentlyleading to lowering of the energy resolution, or increasing the lengthof the flight path by moving the detector well beyond the focus region.

Since the dominant parameter limiting resolving power is the initialenergy spread it is determined that lengthening the flight path is theappropriate solution to increasing total flight time to separate masses.Using a two-field ion source, as shown in FIG. 2B, the space focusregion could be located farther than 2S₀ from the ion source at adistance which is a function of the two accelerating field strengths.Thus, while the low amplitude first accelerating field slightly reducedthe energy resolution, the ability to achieve both space focusing and anincrease in the total flight time for all ions yielded an overallincrease in resolving power.

Another early design provided additional focusing by introducing anadjustable time delay between ion formation and application of anacceleration field (Wiley, W. C., McLaren, I. H., Rev. Sci. Instrumen.1955, 26, 1150-1157; Wiley, W. C., Science, 1956, 124, 817-820; Wiley,W. C. U.S. Pat. No. 2,685,035). During this time, ions move to newlocations in the ion source due to their thermal energies and, uponextraction, acquire total kinetic energies dependent on these newlocation. This energy focusing method, known then as time-lag focusingand now known as pulsed or delayed extraction essentially attempts totransform the energy distribution of the initial ion population into aspatial distribution, thus reducing the temporal effect of the energydistribution at the space focus position. The combined use of time-lagand space focusing yields a significant increase in resolving power.However, the optimal time lag is mass dependent, limiting the m/z rangethat could be simultaneously measured.

Another technique for improving the resolving power is the reflectron orion mirror which provides mass-independent ion focusing (Karataev, V. I.Mamyrin, B. A., Shmikk, D. V. Sov. Phys. Tech. Phys. 1972, 16, 1177.;Mamyrin, B. A., Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov.Phys. JETP 1973,37,45.; Mamyrin, B. A., Shmikk, D. V. Sov. Phys. JETP1979, 49, 762.; Mamyrin, B. A., Karataev, V. I.; Shmikk, D. V. U.S. Pat.No. 4,072,862). An ion mirror in its basic form is shown in FIG. 3A. Ionmirror 30 comprises simply a series of electrostatic diaphragms 32 thatprovide a retarding electric field with enough potential to reflections. Ions with different kinetic energies penetrate the mirror todifferent depths before being turned around and repelled from themirror.

While all ions leave the mirror having exactly the same magnitude ofenergy with which they entered, those ions possessing the greater energytravel farther into the mirror before being repelled and thus experiencea time delay that compensates for their higher velocity in thefield-free region. The ions are then focused at a second space-focusposition SFP2 where they achieve a higher resolving power than the firstspace-focus position SFP1 due to the additional energy focusing. Asshown in FIG. 3B, the original ion mirror design generates a single,linear electric field behind a field isolating mesh 33 and is capable offirst-order focusing. A subsequent design incorporates two fields and iscapable of first or second-order focusing.

Mass spectrometers using linear-field focusing devices such as thetwo-field ion source (shown in FIG. 2B) and the two-field ion mirrorgenerate adequate resolving power for applications having a relativelysmall initial ion energy distribution. However, for applications havinga relatively large initial ion energy distribution, the achievableresolving power is diminished. This is expected since the relationshipbetween energy, velocity and time is fundamentally non-linear, andlinear-field devices provide only an approximation of complete temporalfocusing. Electrospray ionization (ESI) and MALDI, the two majorionization methods used in biological research, both generate ionpopulations having a relatively large energy distribution. One approachto compensate for this, used more commonly with ESI, overcomes thecurrent energy focusing limitation by delivering externally-generatedions to the TOF mass analyzer in a direction orthogonal to the analysisaxis. Thus, while the overall magnitude of initial ion energy isrelatively large, the magnitude along the analysis axis is minimal. ForMALDI-TOF, however, the ionization process occurs within the sourcealong the analysis axis. A large initial ion energy distribution is thusinherent to the analysis, presenting a need for improved focusingmethods.

The fundamentally non-linear relationship between time and energy in ionmotion indicates that the ultimate attainable resolving power can onlybe achieved using non-linear fields, and the development of focusingmethods using such fields is recently building momentum. Several ionmirror designs using a non-linear field have been developed(Glashchenko, V. P.; Semkin, N. D., Sysoev, A. A., Oleinikov, V. A.,Tatur, V. Yu. Sov. Phys. Tech. Phys. 1985, 30, 540-541.; Mamyrin, B. A.Int. J. Mass Spectrom. Ion Processes, 1994, 131, 1-19.; Rockwood, A. L.Proc. 34 ^(th) ASMS Conf. on Mass Spectrom. & Allied Topics, 1986,Cincinnati, Ohio, 173.), while other designs have been proposed and/orpatented (Yoshida, Y. U.S. Pat. No. 4,625,112; Frey, R., Schlag, E. W.,U.S. Pat. No. 4,731,532; Kutscher, R., Grix, R., Li, G., Wollnik, H.,U.S. Pat. No. 5,017,780; Managadze, G. G., Shutyaev, I. Yu. In LaserIonization Mass Spectrometry, Vertes, A., Gijbels, R., Adams, F., Eds.,John Wiley & Sons: New York, 1993, 505-549. Flory, C. A., Taber, R. C.,Yefchak, G. E. Int. J. Mass Spectrom. Ion Proc. 1996, 152, 177-184;Doroshenko, V. M., Cotter, R. J. J. Am. Soc. Mass Spectrom., 1999, 10,992-999; Cotter, R. J., Doroshenko, V. M. U.S. Pat. No. 6,365,892). Allof which are incorporated herein in their entirety by reference.

Each of these designs provides only minor improvement to the resolvingpower achieved using linear-field ion mirrors, and each is suitable toonly a relatively narrow initial range of ion energies. Non-linear-fieldmirrors that focus a broad range of initial ion energies have also beendeveloped using either an entirely gridless design to achieve a singlenon-linear field (Cornish, T. J., Cotter, R. J. Rapid Comm. MassSpectrom., 1993, 7, 1037-1040), or a gridded design generating acombination of linear and non-linear fields (Beussman, D. J., Vlasak, P.R., McLane, R. D., Seeterlin, M. A.; Enke, C. G. Anal. Chem. 1995,67(21), 3952-3957).

While non-linear fields are theoretically preferable to linear fields,one of the drawbacks to generating such fields in ion mirrors is theresult of their inherent radial field-inhomogeneity. Linear fieldsgenerate an electric potential that is constant in all directionsorthogonal to the electric field. Thus, an ion beam entering alinear-field ion mirror at a fixed angle will experience the same forceregardless of the entry point. In contrast, an ion beam entering anon-linear field will experience a force that depends on the exact pointof entry. An ion beam of finite diameter will thus experience a range ofnon-linear fields, which reduces the resultant resolving power andradially disperses the ion beam, diminishing the ion transmission. Anon-linear design has been developed that exploits the radial dispersionusing a single-electrode can-shaped “endcap” ion mirror (Cornish, T. J.,Cotter, R. J. Anal. Chem. 1997, 69(22), 4615-4618; Cornish, T. J.;Cotter, R. J. U.S. Pat. No. 5,814,813). A more recent and somewhat morecomplicated design also uses a minimum number (2 to 3) of electrodes toachieve the desired non-linear field (Zhang, J., Enke, C. G. J. Am. Soc.Mass Spectrom., 2000, 11(9), 759-764; Zhang, J., Gardner, B. D., Enke,C. G. J. Am. Soc. Mass Spectrom., 2000, 11(9), 765-769; Zhang, J.,Gardner, B. D., Enke, C. G., Patent Pending).

In contrast to the developments in non-linear ion mirror design, the useof non-linear fields in ion source design is less prevalent. Severaldesigns have been developed, for the analysis of gas-phase ions, where a“quadratic” non-linear ion-accelerating field is generated (Crane, W.S., Mills, A. P. Rev. Sci. Instrum. 1985, 56, 1723.; Hulett, L. D.,Donohue, D. L., Lewis, T. A. Rev. Sci. Instrum. 1991, 62,2131-2137;Rockwood, A. L., Udseth, H. R., Gao, Q.: Smith, R. D. Proc. 42^(nd) ASMSConf. on Mass Spectrom. & Allied Topics, 1994, Chicago, Ill., 1038). Amass spectrometer based on one of these designs, for the analysis oforthogonally-injected gas-phase ions, is commercially available (LECOCorp., product literature on the Jaguar LC-TOF mass spectrometer).

A separate design incorporating both linear and non-linear fields hasbeen reported (Gardner, B. D., Holland, J. F. J. Am. Soc. MassSpectrom., 1999, 10(11), 1067-1073), also for the analysis of gas-phaseions. A gridless ion source, which consequently generates a non-linearfield by default, is also commercially available on a MALDI-TOFinstrument, although the design has not been described (KratosAnalytical AXIMA).

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a time-of-flight massspectrometer that includes a first electrode, a second electrode spacedapart from the first electrode, and a third electrode arranged betweenthe first and second electrodes. The third electrode reserves a spacefor ions to travel between the first and second electrodes. The massspectrometer further includes a sample probe disposed proximate thefirst electrode and adapted to hold a sample and a detector disposedproximate the second electrode. The first electrode is adapted to beconnected to a voltage source to cause a difference in voltage betweenthe first and second electrodes to provide an electric fieldtherebetween that changes non-linearly along an ion path between thesample probe and the detector for accelerating ions to be detected.

In one embodiment, the first electrode defines a hole therethrough andthe sample probe is disposed within the hole. In this embodiment, thefirst electrode is adapted to provide a beam collimating field in aregion of the hole defined therethrough.

The first and second electrodes can be, for example, substantiallyannular plates, and the third electrode can be, for example, acylindrical electrode and the detector is disposed in an annulus definedby the second electrode. The first, second and third electrodes, and thedetector together provide a mass analyzer that is adapted to provide anelectric field that changes non-linearly along substantially the entirepaths of ions to be detected.

In one embodiment, the second and third electrodes can be adapted to beprovided, for example, with substantially equal electric potentials thatare different from electric potentials of the first electrode and saiddetector during a mode of operation. The second electrode can also beadapted to be provided with, for example, a different electric potentialthan at least one of said detector and said sample probe.

In another embodiment, the mass spectrometer may further comprise afourth electrode spaced apart from the second electrode on a side of thesecond electrode opposite the first electrode, and a fifth electrodedisposed between the second electrode and the fourth electrode. Thefifth electrode reserves a space for passage of ions to be detectedbetween the second and fourth electrodes and detector defines anaperture to permit passage of ions therethrough. The fourth electrode isadapted to be connected to a voltage source to cause a difference involtage between the fourth electrode and the second electrode to providean electric field therebetween that changes non-linearly along an ionpath between the detector and the fourth electrode. The fourth electrodecan be, for example, a substantially circular plate and the fifthelectrode can be, for example, a cylindrical electrode. In thisembodiment, the second, fourth and fifth electrodes together form anon-linear ion mirror that deflects ions that pass through the aperturein the detector to return to and be detected by said detector.

In another embodiment, the first, second and third electrodes can have,for example, convex surfaces arranged so that they can be used in an iontrap configuration.

In one embodiment, the mass spectrometer can include a laser arranged togenerate ions from a sample when held by the sample probe. In anotherembodiment the mass spectrometer can further include a source of a timevarying electric potential connected to the sample probe to provide apulsed source electric potential.

Another aspect of the present invention is to provide a method ofmeasuring the mass-to-charge ratio of an ion, the method includesgenerating an electric field between a sample region and a detector thatchanges non-linearly with position therebetween, injecting the ion intothe electric field to be accelerated to the detector, and detecting theion and determining a time of flight of the ion. The method may furtherinclude generating the ion from a sample by irradiating the sample witha laser. The method may also include generating an electric field todecelerate and then accelerate the ion in a direction reversed from aninitial direction prior to said detecting the ion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the invention will become moreapparent and more readily appreciated from the following detaileddescription of the presently preferred exemplary embodiments of theinvention, taken in conjunction with the accompanying drawings, ofwhich:

FIG. 1A is a schematic representation of a conventional time-of-flightspectrometer;

FIG. 1B is a linear electrical potential profile applied in the ionsource of a time-of-flight spectrometer of FIG. 1A;

FIG. 2A is a linear electrical potential profile and its relation to thespace focus position of the ions;

FIG. 2B is a two-field electrical potential profile and its relationwith the space focus region;

FIG. 3A is a schematic representation of a conventional ion mirror;

FIG. 3B is a retarding electric field applied in the ion mirror shown inFIG. 3A;

FIG. 4A is a schematic representation of a non-linear time-of-flightmass spectrometer according to an embodiment of the present invention;

FIG. 4B is a 3-dimensional topographical view of the physical geometryand non-linear electrical field distribution in the mass spectrometer ofFIG. 4A;

FIG. 5 is a schematic representation of a non-linear time-of-flight massspectrometer using a non-linear electrical field ion mirror according toanother embodiment of the present invention; and

FIG. 6 is a schematic representation of a non-linear time-of-flight massspectrometer using a non-linear electrical field in an ion trapgeometry.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION

One aspect of the present invention is to provide a mass spectrometer inwhich substantially the entire flight path of ions uses non-linearelectric fields for ion acceleration and temporal focusing.

One embodiment of a mass spectrometer according to the present inventionis shown schematically in FIG. 4A. Mass spectrometer 40 is atime-of-flight spectrometer comprising ion source (sample probe) 41, iondetector 42, and electrode 43 having opening 44 to accommodate ionsource 41. The mass spectrometer 40 further comprises electrode 45arranged substantially perpendicularly to electrode 43 and electrode 46arranged substantially perpendicularly to electrode 45. The electrode 46can be arranged substantially parallel to electrode 43 but separatedfrom electrode 43 by a distance substantially equal to at least thelength of electrode 45. The electrode 46 has an opening 47 configured tohold ion detector 42.

In one embodiment the electrode 43 has a ring or annular shape with theopening in the middle corresponding to opening 44 and electrode 45 has acylindrical shape. A diameter of the electrode 45 can be substantiallyequal to the external diameter of electrode 43. However, one of skill inthe art would appreciate that different shapes of the various electrodesare also within the scope of the present invention. For example, theelectrode 43 may have a polygonal shape with an opening in its centerand the electrode 45 can have an ellipsoid shape (a tube with anelliptical base) or a tube with a polygonal base or other variations.

In this embodiment, the ion source 41 is surrounded by walls 44W ofopening 44. For the analysis of positively charged ions, the electrode43 is held at high potential, for example 18 kV, while the electrode 46attaching the detector 42 is held at low voltage, for example 0 volt:The cylindrical electrode 45 can be held at any intermediate voltage,with the voltage being selected to optimize the resulting mass resolvingpower. The ion source sample probe 41 can be held at a potentialrelatively equal to the potential of electrode 43. Since the electrode45 is held at a different voltage than the electrode 43, the electrode45 is electrically decoupled from electrode 43 to allow the onset of apotential difference between the electrode 43 and electrode 45. Theelectrode 46, on the other hand, can be electrically coupled toelectrode 45. For example, as illustrated in FIG. 4A, electrode 45 andelectrode 46 arc both held at the same potential (0 volt or groundpotential).

The electrode 43 being held at a high potential, for example 18 kV andthe electrode 46 being held at low voltage, for example 0 volt, allowsthe onset of a non-linear electrical field between these electrodes, asshown in FIG. 4A with the iso-potential lines of electrical field and inFIG. 4B in the 3-dimensional topographical view of the physical geometryand non-linear electrical field distribution. In particular, a shallownon-linear electric field forms in the source region between the ionsource 41 and the exit of opening 44 of electrode 43. This shallownon-linear electric field serves as an ion beam focusing lens thatfocuses the ions generated at the ion source (sample probe) 41 to afocal point relatively in the vicinity of the exit of opening 44.

The detector 42 can be selected from any commercially available chargedparticles detector. Such detectors include, but are not limited to, anelectron multiplier, a channeltron or a micro-channel plate (MCP)assembly. Although, a micro-channel plate is shown as the detector inFIG. 4A, one skilled in the art would readily understand that usingother detectors are also within the scope of the present invention.

An electron multiplier is a discrete dynode with a series of curvedplates facing each other but shifted from each such that an ion strikingone plate creates secondary electrons and an effect of electronavalanche follows through the series of plates. A channeltron is ahorn-like continuous dynode structure that is coated on the inside withan electron emissive material. An ion striking the channeltron createssecondary electrons that have an avalanche effect to create moresecondary electrons and finally a current pulse.

A microchannel plate is made of a leaded-glass disc that containsthousands or millions of tiny pores etched into it. The inner surface ofeach pore is coated to facilitate releasing multiple secondary electronswhen struck by an energetic electron or ion. When an energetic particlesuch as an ion strikes the material near the entrance to a pore andreleases an electron, the electron accelerates deeper into the porestriking the wall thereby releasing many secondary electrons and thuscreating an avalanche of electrons.

In most applications, two channel plates are assembled to provide anincreased gain of electrons. In the embodiment shown in FIG. 4A, a MCPassembly is used as the ion detector 42. The detected electron signalcorresponding to an ion striking the detector is further amplified,integrated, digitized and recorded into a memory for later analysisand/or displayed through a graphical interface for evaluation.

For example, in MALDI, the ions are formed by ionizing a sample in thesample probe with a laser. In this instance, the mass spectrometer isprovided with a laser which can be pulsed or continuous and the light isdirectly focused on the sample with either an optical system usinglenses, prisms, etc. or directed through an optical fiber to the sample.

The mass spectrometer 40 consists of detecting the arrival of the ionsat the detector 42 and measuring their time-of-flight in reference to,for example, firing the laser pulse or the application of a voltagepulse to the sample plate 41. Since, as explained above, thetime-of-flight is proportional to the square root of the mass of theions, knowing the time-of-flight allows the determination of the mass ofthe ions and thus the identification of the ions.

Upon laser ionization of the sample 41 on the sample surface, thegenerated ions may be immediately ejected into the main body of the massanalyzer, where the time-dependent mass separation occurs.Alternatively, after a short, definable delay subsequent to laserionization, a voltage pulse may be applied to the sample electrode toeject the ions into the mass analyzer. The voltage pulse applied to thesample probe or plate 41 may be delayed relative to the laser pulse toincrease efficiency of ion extraction. The mass spectrometer can beconfigured to detect either positively or negatively charged ions.

In another embodiment shown in FIG. 5, the mass spectrometer 50comprises the same elements as the mass spectrometer 40 and furthercomprises ion mirror 51 to provide additional energy focusing to theions. Specifically, mass spectrometer 50 comprises ion source (sampleprobe) 41, electrode 43 having opening 44 to accommodate ion source 41,electrode 45 arranged substantially perpendicularly to electrode 43 andelectrode 46 arranged substantially perpendicularly to electrode 45. Theelectrode 46 holds ion detector 52. In this embodiment of the massspectrometer, the ion detector 52 has a hole or an aperture in itscenter configured to allow the ions to enter the ion mirror 51.

The ion mirror “endcap” 51 includes electrode 53 and electrode 54.Electrode 53 is held at some high potential enough to reverse thetrajectory of the ions. The electrode 53 can be held at a potentialslightly greater than the potential of electrode 43 to enable reflectionof the ions. For example, if the electrode 43 is held at a potential of18 kV, the electrode 53 can be held at 19 kV. Similarly to electrode 45,the electrode 54 can be held to some lower potential. For example,electrode 54 can be held at a potential of 0 volt.

Due to the difference of potential existing between electrode 53 andelectrode 46 and due to the difference of potential existing between theelectrode 53 and electrode 54 a non-linear electrical field isestablished, thus allowing further energy focusing in addition toreflecting the ions back into ion detector 52 as illustrated on FIG. 5.

Although the mass spectrometer 50 is described using specifically ionmirror 51, the mass spectrometer 50 can also perform temporal focusingby using, for example, the ion mirror 30 described above and shown inFIG. 3A. Similarly the mass spectrometer 50 can perform temporalfocusing by using ion mirror 51 coupled with a conventional ion massanalyzer such as the mass analyzer shown in FIG. 1A and described above.

In another embodiment shown in FIG. 6, the time-of-flight massspectrometer 60 comprises ion source sample probe 62, ion detector 63, afirst end cap electrode 64 arranged proximate to ion source 62, and asecond end cap electrode 65 arranged proximate detector 63. The massspectrometer 60 further comprises a ring electrode 66 arranged betweenthe first end cap electrode 64 and the second end cap electrode 65. Thefirst end cap electrode 64 and second end cap electrode 65 have,respectively, openings 64A and 65A for allowing the ions to travel fromthe sample probe 62 to the ion detector 63.

The ring electrode 66 may be connected to either a radio-frequencyvoltage source and the mass spectrometer operates in ion trap mode or toa constant voltage and the mass spectrometer operates in time-of-flightmode. In the same fashion, the first end cap electrode 64 and second endcap electrode 65 may be selectively connected to either a supplementalradio-frequency voltage source when the mass spectrometer operates inion trap mode or to constant voltage source when the spectrometeroperates in time-of-flight mode. A detailed description of operation ofthe mass spectrometer is given in a co-pending application entitled“Combined Chemical/Biological Agent Mass Spectrometer Detector,”Attorney Docket Number 41061/302302, the entire contents of which areherein incorporated by reference.

In this instance, the mass spectrometer is operated in time-of flightmode and in this mode of operation, first end cap electrode 64 isconnected to a voltage potential. Whereas, second end cap electrode 65is maintained at a constant voltage substantially equal to the constantvoltage applied to ring electrode 66. In this way, a non-linearelectrical field is generated between the first end cap electrode 64 andthe ring electrode 66 and between the first end cap electrode 64 andsecond end cap electrode 65.

Therefore,.similarly to time-of-flight mass spectrometer 40, due to thepresence of the non-linear electrical field, energy focusing occurs inthe ion beam.

Although the mass spectrometer of the present invention is shown invarious specific embodiments, one of ordinary skill in the art wouldappreciate that variations to these embodiments can be made thereinwithout departing from the spirit and scope of the present invention.For example, although the mass spectrometer has been described with theuse of a laser as an ionizing source, one of ordinary skill in the artwould appreciate that using electron ionization, electrospray,atmospheric pressure ionization (API) and atmospheric MALDI (APMALDI) isalso within the scope of the present invention. The many features andadvantages of the present invention are apparent from the detailedexemplary embodiments and the scope is determined by the appendedclaims.

Furthermore, since numerous modifications and changes will readily occurto those of skill in the art, it is not desired to limit the inventionto the exact construction and operation described herein. Moreover, theprocess and apparatus of the present invention, like related apparatusand processes used in mass spectrometry arts tend to be complex innature and are often best practiced by empirically determining theappropriate values of the operating parameters or by conducting computersimulations to arrive at a best design for a given application.Accordingly, all suitable modifications and equivalents should beconsidered as falling within the spirit and scope of the invention.

1. A time-of-flight mass spectrometer, comprising: a first electrode; asecond electrode spaced apart from said first electrode; a thirdelectrode arranged between said first and second electrodes, said thirdelectrode reserving a space for ions to travel between said first andsecond electrodes; a sample probe disposed proximate said firstelectrode and adapted to hold a sample; and a detector disposedproximate said second electrode, wherein said first electrode is adaptedto be connected to a voltage source to cause a difference in voltagebetween said first and second electrodes to provide an electric fieldtherebetween that changes non-linearly along an ion path between saidsample probe and said detector for accelerating ions to be detected. 2.A time-of-flight mass spectrometer according to claim 1, wherein saidfirst electrode defines a hole therethrough, said sample probe beingdisposed within said hole, wherein said first electrode is adapted toprovide a beam collimating field in a region of said hole definedtherethrough.
 3. A time-of-flight mass spectrometer according to claim2, wherein said first and second electrodes are substantially annularplates, and said third electrode is a cylindrical electrode, saiddetector being disposed in an annulus defined by said second electrode.4. A time-of-flight mass spectrometer according to claim 3, wherein saidfirst, second and third electrodes, and said detector together provide amass analyzer that is adapted to provide an electric field that changesnon-linearly along substantially entire paths of ions to be detected. 5.A time-of-flight mass spectrometer according to claim 3, furthercomprising: a fourth electrode spaced apart from said second electrodeon a side of said second electrode opposite said first electrode; and afifth electrode disposed between said second electrode and said fourthelectrode and reserving a space for passage of ions to be detectedbetween said second and fourth electrodes, wherein said detector definesan aperture to permit passage of ions therethrough, and wherein saidfourth electrode is adapted to be connected to a voltage source to causea difference in voltage between said fourth electrode and said secondelectrode to provide an electric field therebetween that changesnon-linearly along an ion path between said detector and said fourthelectrode.
 6. A time-of-flight mass spectrometer according to claim 5,wherein said fourth electrode is substantially a circular plate and saidfifth electrode is a cylindrical electrode.
 7. A time-of-flight massspectrometer according to claim 5, wherein said second, fourth and fifthelectrodes together form a non-linear ion mirror that deflects ions thatpass through said aperture in said detector to return to and be detectedby said detector.
 8. A time-of-flight mass spectrometer according toclaim 2, wherein said first, second and third electrodes have convexsurfaces arranged so that they can be used in an alternative ion trapconfiguration.
 9. A time-of-flight mass spectrometer according to claim3, wherein said second and third electrodes and said detector areadapted to be provided with substantially equal electric potentials thatare different from electric potentials of said first electrode and saidsample probe during a mode of operation.
 10. A time-of-flight massspectrometer according to claim 3, wherein said second and thirdelectrodes and said sample probe are adapted to be provided withsubstantially equal electric potentials that are different from electricpotentials of said first electrode and said detector during a mode ofoperation.
 11. A time-of-flight mass spectrometer according to claim 3,wherein said second electrode is adapted to be provided with a differentelectric potential than at least one of said detector and said sampleprobe.
 12. A time-of-flight mass spectrometer according to claim 1,further comprising a laser arranged to generate ions from a sample whenheld by said sample probe.
 13. A time-of-flight mass spectrometeraccording to claim 1, wherein an electric field proximate said sampleprobe changes non-linearly along an ion path to said detector.
 14. Atime-of-flight mass spectrometer according to claim 1, furthercomprising a source of a time varying electric potential connected tosaid sample probe to provide a pulsed source electric potential.
 15. Atime-of-flight mass spectrometer according to claim 1, wherein saidsecond and third electrodes are connected to form a single electrode.16. A method of measuring the mass-to-charge ratio of an ion comprising:generating an electric field between a sample region and a detector thatchanges non-linearly with position therebetween; injecting said ion intosaid electric field to be accelerated to said detector; and detectingsaid ion and determining a time of flight of said ion.
 17. A method ofmeasuring the mass-to-charge ratio of an ion according to claim 16,further comprising generating said ion from a sample by irradiating saidsample with a laser.
 18. A method of measuring the mass-to-charge ratioof an ion according to claim 16, further comprising generating said ionfrom a sample by applying a pulsed electric potential to said sample.19. A method of measuring the mass-to-charge ratio of an ion accordingto claim 16, further comprising: generating an electric field todecelerate and then accelerate said ion in a direction reversed from aninitial direction prior to said detecting said ion.
 20. A method ofmeasuring the mass-to-charge ratio of an ion according to claim 19,wherein said electric field generated to decelerate and then acceleratesaid ion changes non-linearly along a path of said ion.